The present invention relates to a control Valve used in a variable displacement compressor. More particularly, the present invention pertains to a control valve that controls the compressor displacement by adjusting the pressure in a crank chamber.
A typical refrigerant circuit of a vehicle air conditioner includes a condenser, an expansion valve, an evaporator and a compressor. The compressor receives refrigerant gas from the evaporator. The compressor then compresses the gas and discharges the gas to the condenser. The evaporator transfers heat to the refrigerant in the refrigerant circuit from the air in the passenger compartment. The pressure of refrigerant gas at the outlet of the evaporator, in other words, the pressure of refrigerant gas that is drawn into the compressor (suction pressure Ps), represents the thermal load on the refrigerant circuit.
Variable displacement swash plate type compressors are widely used in vehicles. Such compressors include a displacement control valve that operates to maintain the suction pressure Ps at a predetermined target level (target suction pressure). The control valve changes the inclination angle of the swash plate in accordance with the suction pressure Ps for controlling the displacement of the compressor. The control valve includes a valve body and a pressure sensing member such as a bellows or a diaphragm. The pressure sensing member moves the valve body in accordance with the suction pressure Ps, which adjusts the pressure in a crank chamber. The inclination of the swash plate is adjusted, accordingly.
In addition to the above structure, some control valves include an electromagnetic actuator, such as a solenoid, to change the target suction pressure. An electromagnetic actuator urges a pressure sensing member or a valve body in one direction by a force that corresponds to the value of an externally supplied current. The magnitude of the force determines the target suction pressure. Varying the target suction pressure permits the air conditioning to be finely controlled.
Such compressors are usually driven by vehicle engines. Among the auxiliary devices of a vehicle, the compressor consumes the most engine power and is therefore a great load on the engine. When the load on the engine is great, for example, when the vehicle is accelerating or moving uphill, all available engine power needs to be used for moving the vehicle. Under such conditions, to reduce the engine load, the compressor displacement is minimized. This will be referred to as a displacement limiting control procedure. A compressor having a control valve that changes a target suction pressure raises the target suction pressure when executing the displacement limiting control procedure. Then, the compressor displacement is decreased such that the actual suction pressure Ps is increased to approach the target suction pressure.
The graph of FIG. 11 illustrates the relationship between suction pressure Ps and displacement Vc of a compressor. The relationship is represented by multiple lines in accordance with the thermal load in an evaporator. Thus, if the suction pressure Ps is constant, the compressor displacement Vc increases as the thermal load increases. If a level Ps1 is set as a target suction pressure, the actual displacement Vc varies in a certain range (xcex94Vc in FIG. 11) due to the thermal load. If a high thermal load is applied to the evaporator during the displacement limiting control procedure, an increase of the target suction pressure does not lower the compressor displacement Vc to a level that sufficiently reduces the engine load.
Thus, the compressor displacement is not always controlled as desired as long as the displacement is controlled based on the suction pressure Ps.
Accordingly, it is an objective of the present invention to provide a control valve used in a variable displacement compressor that accurately controls the compressor displacement regardless of the thermal load on an evaporator.
To achieve the above objective, the present invention provides a control valve for a variable displacement compressor used in a refrigerant circuit. The refrigerant circuit includes a condenser and a high pressure passage extending from a discharge chamber of the compressor to the condenser. A section of the refrigerant circuit that includes the discharge chamber, the condenser and the high pressure passage forms a high pressure zone. The control valve controls the pressure in a crank chamber of the compressor to change the displacement of the compressor. The control valve includes a valve housing. The valve housing is located in a supply passage, which connects the high pressure zone to the crank chamber, The supply passage includes an upstream section, which is between the high pressure zone and the valve housing, and a downstream section, which is between the valve housing and the crank chamber. A first pressure chamber is defined in the valve housing. The first pressure chamber is exposed to the pressure of a first pressure monitoring point, which is located in the high pressure zone. A second pressure chamber is defined in the valve housing. The second pressure chamber is exposed to the pressure of a second pressure monitoring point, which is located in a part of the high pressure zone that is downstream of the first pressure monitoring point. The upstream section of the supply passage connects the first pressure chamber or the second pressure chamber to the corresponding pressure monitoring point. A valve body is located in the valve housing. The valve body adjusts the opening size of the supply passage. A pressure receiving body is located in the valve housing. The pressure receiving body moves the valve body in accordance with the difference between the pressure in the first pressure chamber and the pressure in the second pressure chamber.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.